There is mounting evidence for mitochondrial involvement in many neurodegenerative diseases. In considering the cellular targets for environmental agents, mitochondrial DNA (mtDNA) is extremely vulnerable. Mutations in mtDNA, whether inherited or acquired, lead to impaired electron transport chain functioning and cellular dysfunction. For this competitive renewal, the objective will continue to be elucidation of the mechanisms by which cells of the central nervous system, particularly oligodendrocytes, deal with environmental insults. The hypothesis to be tested is that an elevation in the lesion equilibrium in mtDNA is an early key factor that sets in motion the cascade of events that ultimately leads to cellular dysfunction and death. This hypothesis will be tested through four specific aims. The first aim is to determine how different populations of glial cells respond to damage to mtDNA caused by reactive nitrogen species (RNS). These studies will use primary cultures of oligodendrocytes and astrocytes to test the hypothesis that the ability of cells to deal with damage to their mitochondrial genomes is one of the key factors that contributes to the susceptibility of different populations of glial cells to RNS. The second aim is to determine how unrepaired mtDNA damage affects mtDNA transcription and ATP production. These studies will answer questions relating to the hypothesis that the inability of oligodendrocytes to repair lesions in their mtDNA results in decreased mtDNA transcription and subsequent reduction in the generation of ATP. The third aim is to assess how mtDNA repair influences glial cell function. These studies will evaluate factors that may influence the maintenance of an acceptable lesion equilibrium in mtDNA. The fourth aim is to evaluate the effects of recombinant DNA repair proteins targeted to the mitochondria. These studies will use primary cultures of astrocytes and oligodendrocytes that are transfected with genes that encode mtDNA repair enzymes to modulate mtDNA repair capacity and alter the lesion equilibrium in order to directly test the hypothesis that elevations in the lesion equilibrium in mtDNA act as a early key factor that ultimately leads to cellular dysfunction and death. The research seeks to provide an enhanced understanding of the role that damage to mtDNA and its subsequent repair play in the pathogenesis of neurodegenerative diseases.